This invention relates generally to RF power amplifier circuits, and more particularly the invention relates to an RF power amplifier circuit suitable for modern wireless communication systems, which require a wide range of output power in basestations where digital modulation is being employed.
Power amplifiers in basestations often operate at output power levels much lower than peak power. Unfortunately, the back-off power level reduces the efficiency of the power amplifier in the transmitter. In a conventional amplifier there is a direct relationship between efficiency and the input drive level. Therefore, high efficiency (DC to RF conversion efficiency) is not obtained until the RF input power level becomes sufficiently high to drive the amplifier into compression or saturation. Since in multicarrier communication systems an amplifier must remain as linear as possible, this region of high efficiency cannot be used.
A power amplifier circuit design which provides improved efficiency in back-off power levels is the Doherty amplifier circuit, which combines power from a main or carrier amplifier and from an auxiliary or peak amplifier. See, W. H. Doherty, xe2x80x9cA New High-Efficiency Power Amplifier for Modulated Waves,xe2x80x9d Proc. IRE Vol. 24, No. 9, pp. 1163-1182 1936. In the conventional Doherty configuration, the carrier amplifier 10 and peak amplifier 12 are designed to deliver maximum power with optimum efficiency to a load R, as shown in FIG. 1A. The main or carrier amplifier is a normal Class B amplifier, while the peak amplifier is designed to only amplify signals which exceed some minimum threshold. For an LDMOS power transistor, this can be accomplished by DC biasing the transistor below its pinch-off voltage for operation similar to Class C. The outputs of the two amplifiers are connected by a quarter-wave transmission line of characteristic impedance R, and a load of one-half of the optimum load R is attached to the output of the peak amplifier. The RF input power is divided equally with a quarter-wave delay at the input to the peak amplifier, thus assuring that the output power of the two amplifiers at the load R/2 will be in phase.
The Doherty amplifier circuit achieves high efficiency prior to compression by operating the Class B carrier amplifier into an apparent load impedance two times larger than its optimum load. (Before the peak amplifier becomes active, the apparent load impedance presented to the carrier amplifier is 2R due to the presence of quarter wave transformer 14.) Thus, the carrier amplifier compresses and reaches peak efficiency at half of its maximum power. The second or peak amplifier becomes active only during the peaks of the input signal. When the peak amplifier is active, the load impedance apparent at the output of the carrier amplifier is reduced. Maximum efficiency is again achieved when the second amplifier puts out its full power. Thus, the first amplifier is kept on the verge of saturation for a 6 dB range of output power and near-peak efficiency can be maintained.
When the input RF power into the Doherty amplifier circuit is not sufficient to turn on the peak amplifier, all of the output power is supplied by the main or carrier amplifier. When the peak amplifier is off, its output impedance is very high and the output power of the carrier amplifier is entirely delivered to load R/2, as shown in FIG. 1B. As discussed above, the load actually presented to the carrier amplifier across the quarter-wave transformer 14 is 2R. The device current is therefore one-half of what is delivered at maximum power while the voltage is saturated. This results in the device delivering half its maximum output power. Since both the RF and DC components of the current are half their peak values, the efficiency will be at its maximum with half of the maximum output power of the carrier amplifier being supplied to the load with maximum linear efficiency.
When sufficient input RF power is provided to allow the peak amplifier to become saturated, as in FIG. 1A, two parallel amplifiers are evenly delivering maximum output power to the load R/2. The load apparent to each amplifier is then the optimum load R, and the load at both ends of the quarter-wave transformer will remain at R. The peak amplifier is designed to begin operation when the carrier amplifier just begins to saturate. Maximum linear efficiency is obtained at this point. As the input RF drive is further increased, the peak amplifier begins to turn on and deliver output power to the load. The additional current supplied by the peak amplifier has the effect of increasing the load impedance at the output of the quarter-wave transformer. The effective change at the carrier amplifier end of the transformer will be a reduction in the apparent load impedance and enabling the carrier amplifier to deliver more power while its voltage remains saturated. The efficiency between the limits will fall off only slightly from the maximum since the duty factor of the peak amplifier is relatively low.
Attempts have been made to extend the range of high efficiency operation of the Doherty amplifier circuit. For example, Iwamoto et al. have produced a 12 dB back-off circuit using scaled transistors or different sized transistors in the carrier and peak amplifiers and an unequal power splitter at the input. See, Iwamoto et al., xe2x80x9cAn Extended Doherty Amplifier with High Efficiency Over a Wide Power Range,xe2x80x9d 2001 IEEE MTT-S Digest, Phoenix, Ariz. This technique apparently works well when the total output power is low (less than 1 watt), but with limited improvement when the output power is in the 10 to 100 watt CW range.
There continues to be a need to extend the range of high efficiency operation for an RF power amplifier.
In accordance with the invention, an RF power amplifier circuit includes a carrier amplifier for maximum back-off power operation and one or more peak amplifiers which are suitably biased to begin operation sequentially at increased power levels. Each peak amplifier can provide an increase of 6 dB in the power range over which the peak efficiency will be maintained. Since an N-way splitter is required for providing an input signal to the carrier amplifier and Nxe2x88x921 peak amplifiers, a finite loss of power in the splitter may limit some of the improvements in efficiency that can be realized. However, the use of peak amplifiers in high input power conditions may improve the overall efficiency of the circuit.
In one embodiment, a four-way amplifier circuit is provided and includes a carrier amplifier and three peak amplifiers all driven by a four-way power splitter. Theoretically, this amplifier may extend the range of efficient power by 18 dB. Such extension in efficient power range is very important in digital communication systems using modulation schemes such as wideband CDMA (W-CDMA) or OFDM where the peak to average power ratios can be as high as 13 dB. The four-way configuration may also provide an overall power increase of 3 dBm compared to a two-way amplifier arrangement. Thus a 120 watt peak amplifier can be provided by a four-way arrangement with each amplifier path (a carrier and three peak amplifiers) utilizing 30 watt transistors.
In accordance with another embodiment of the invention, input power is split and applied unequally to the carrier and one or more peak amplifiers to realize increased power added efficiency (PAE) and linearity. In one embodiment, each peak amplifier is provided an input signal level that is higher than the input signal level provided to the carrier amplifier. The peak amplifier can be pumped by the RF signal more efficiently than can be achieved using an equal power split, thus compensating for transconductance characteristics of the transistor close to threshold and increasing back-off capability for the same efficiency or improved linearity at the same back-off point. In a four-way Doherty arrangement, for example, four different RF power levels can be provided to the carrier and three peak amplifiers. The unequal split provides enhanced performance in back-off efficiency and linearity. In another embodiment, each peak amplifier is provided an input signal level that is less than the input signal level provided to the carrier amplifier.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.